1. Field of the Invention
The present invention relates to longitudinally coupled resonator type surface acoustic wave filters. More particularly, the present invention relates to longitudinally coupled resonator type surface acoustic wave filters having balance-unbalance conversion functions.
2. Description of the Related Art
Recently, the sizes and weights of mobile phones have been greatly reduced. With such a trend, the numbers and sizes of components used in mobile phones have also been reduced while making such components more and more multifunctional.
With the above-described considerations, various kinds of mobile phones have been provided. In these mobile phones, surface acoustic wave filters are incorporated in the RF stages of the phones to have balance-unbalance conversion functions, or so-called balun functions.
FIG. 22 is a schematic plan view for illustrating the electrode structure of a conventional surface acoustic wave filter having a balance-unbalance conversion function.
In this filter, first, second and third IDTs 101, 102 and 103 are arranged in a surface acoustic wave propagating direction. Reflectors 104 and 105 are arranged on each side of the surface acoustic wave propagating direction in the region where the IDTs 101, 102 and 103 are arranged. When a wavelength determined by the electrode finger pitch of each of the IDTs 101, 102 and 103 is λI, both a distance between the centers of the mutually adjacent electrode fingers of the IDTs 101 and 102 and a distance between the centers of the mutually adjacent electrode fingers of the IDTs 102 and 103 are 0.75 λI. The widths of the electrode fingers 109 and 110 of the ends of the IDT 102 are increased to reduce spaces between the IDTs. As a result, loss due to the irradiation of a bulk wave can be reduced. In FIG. 22, terminals 106 and 107 are balanced signal terminals, and a terminal 108 is an unbalanced signal terminal.
In such a surface acoustic wave filter having the balance-unbalance conversion function, regarding propagation characteristics in a pass band between the unbalanced signal terminal 108 and the balanced signal terminal 106 and between the unbalanced signal terminal 108 and the balanced terminal 107, amplitude characteristics need to be equal and propagating signals need to be 180° out of phase with respect to each other. The condition in which the amplitude characteristics are equal is referred to as amplitude balance and the degree at which the propagating signals are 180° out of phase with respect to each other is referred to as phase balance.
The amplitude balance and the phase balance are defined as follows when the surface acoustic wave filter having the balance-unbalance conversion function is regarded as a device having three ports, such as the unbalanced input terminal as port 1 and the balanced output terminals as ports 2 and 3.
Amplitude balance=|A|.
A=|20 log S21|−|20 log S31|.
Phase balance=|B−180|.
B=|∠S21−∠S31|.
In this definition, S21 denotes a transfer factor from the port 1 to the port 2 and S31 denotes a transfer factor from the port 1 to the port 3.
Ideally, in the pass band of the filter, the amplitude balance needs to be 0 dB and the phase balance needs to be 0°. However, with the use of the structure shown in FIG. 22, when intending to obtain a filter having a balance-unbalance conversion function, since the IDT 102 has an odd number of electrode fingers, the number of electrode fingers connected to the balanced signal terminal 106 increases by one more than the number of electrode fingers connected to the balanced signal terminal 107. As a result, there is a problem in that the degree of balance is deteriorated. The higher the central frequency of the filter is, the more noticeable the deterioration. Thus, like DCS filters and PCS filters, in a filter having a central frequency near 1.9 GHz, sufficient balance cannot be obtained.